1. Field of the Invention
The present invention relates to polycrystalline silicon, and more particularly, to polycrystalline silicon that can suitably form a good polycrystalline silicon thin film by growing crystal grains with a constant direction and size, when an amorphous silicon thin film disposed on an insulating film such as a glass substrate is crystallized to a polycrystalline silicon thin film, and a crystallization method thereof.
2. Description of the Prior Art
Usually, most of transistor elements using polycrystalline silicon are mainly employed for the active elements of active-matrix liquid crystal displays (AMLCDs), switching elements of electro-luminescence elements, and peripheral circuits.
In this time, the polycrystalline silicon used as a semiconductor active layer is mainly manufactured by using a direct deposition [Jang et al., Vacuum Vol. 51, 769(1998)], a solid Phase Crystallization [Girginoudi et al., J. Appl. Phys. Vol. 84, 1968(1998)], and a laser heat treatment [J. S. Im and H. J. Kim, Appl. Phys. Lett. Vol. 64, 2303(1994)].
The methods except for the direct deposition obtain polycrystalline silicon by inducing a phase transformation through the heat treatment of amorphous silicon disposed on a substrate. In this time, polycrystalline silicon with a structurally and electrically good thin-film property may be obtained when the amorphous silicon is crystallized while being fused and then cooled. As such, the method of forming polycrystalline silicon with a structurally and electrically good thin-film property may be classified into an excimer laser annealing (ELA) method and a sequential lateral solidification (SLS) method.
The ELA method for generating a laser beam in the form of pulses has been widely used to crystallize amorphous silicon because of its good electrical property. The excimer laser may generate a pulsed beam by performing a high-voltage discharge in mixed gases that mix Helium (He) or Neon (Ne) to halogen gases and inert gases. The excimer laser re-crystallizes the amorphous silicon by radiating a short pulse type of laser whose wavelength range lies within that of ultraviolet ray. The crystallization method using the excimer laser has a good property without any lattice defects. On the other hand, the crystallization method, however, requires expensive equipment and has some problems such as difficulty of large-sized area process and the instability of yield, resulting from the instability of a laser.
To compliment the problems, the SLS method, which adjusts the directionality of crystal grains while moving a mask with width of more or less than 5 μm, has been used. However, the SLS method uses expensive equipment, such as an excimer laser, and has to perform complex processes.
To overcome the problems of afore-mentioned methods, a crystallization method using a CW-DPSS (continuous wave diode pumped solid state) laser with a wavelength range of visible light (532 nm), has been recently introduced. This method is similar to the existing methods, considering that an amorphous silicon thin film is melt and solidified using a high power laser. This method has also more similarity to a zone melting recrystallization (ZMR) method [L. Pfeiffer et al., Appl. Phys. Lett. Vol. 51, 1256(1987)], considering that melting silicon continuously exists inside the thin film by a continuously generated laser beam, and a continuous lateral crystallization is induced through the movement of the laser.
The polycrystalline silicon thin film using the CW laser may include directional grains having directionality because the crystallization is performed in parallel with the moving direction of the laser. This method has been popular as a next-generation method of forming high capacity and low temperature polycrystalline silicon [A. Hara et al., AMLCD'01, Tokyo, Japan, pp 227 (2001)], because of lower installation and maintenance costs as well as simpler process than the SLS method.
Nevertheless, this conventional method suffers from the difficulty in adjusting the size and location of crystal grains, which is one of problems in the crystallization method using a CW laser.